I recommend against this as doing so could allow the bottom of the legs to twist out from under the tank over time. If you need a larger opening, you could just make the stand taller.
Ok, so that leads to another idea. In the original post, you said the uprights on the narrow side are optional. If I remove one or both of them, would the stand still be stable enough for a 180? I would like to have a sump extending through the edges 12", with the skimmer and socks in that area.
Of course I will frame in the area where the sump is extending, and also would like bookshelves on top of that area.
But with the 2x4 uprights on the narrow side, I only have 15" of clearance. I would need 18½" of clearance minimum, which I could achieve by removing one of the narrow uprights. But this way doesn't allow me to put walls inside this area, which is why I want to remove both narrow uprights.